[Feature] Restore ability to inject JS into store, community, etc pages.

[Plaer1]

Before Requesting

• [X] I found no existing issue matching my feature request
• [X] This request is related to Millennium, and not a request for a specific theme/addon

Describe the feature you'd like!

I still need it for some theme-specific features, for example; I'm pretty sure nobody but me will need a plugin that makes the background color on game pages this specific shade of gray:

Anything else?

The only alternatives I can see are:

• I make one big plugin with a lot of theme specific features that probably nobody but me would use, like a library plugin for my theme.
• Try to rebrand every feature as something other people can use (make a variable for changing the backgrounds to other hues, etc)